1. Field of the Invention
The invention is directed to electrical connectors and, more particularly, to the field of coaxial type feed-through RF connectors that require hermetic sealing.
2. Description of the Prior Art
The prior art will be discussed in conjunction with FIGS. 1-3.
FIG. 1 is a diagrammatic cross-sectional illustration of a conventional coaxial feed-through RF connector, having an RF signal ground-providing Kovar shell that projects outwardly beyond the surface of an aluminum housing having a bore in which the RF connector is soldered. For the RF connector shown in FIG. 1, as well as the connectors of FIGS. 2-5, to be described, it is to be understood that each of the illustrated components thereof is cylindrically symmetrical about that connector's longitudinal axis. The RF connector of FIG. 1 includes a longitudinal signal pin 10, which lies along a longitudinal axis 12 of the connector, and has a first, generally central portion 11 hermetically bonded to a coaxial bore 21 of a generally cylindrical dielectric (typically glass) spacer 20.
The outer cylindrical surface of the glass dielectric spacer 20 is contiguous with and hermetically bonded to a reduced diameter portion 31 of a surrounding conductive (metallic) shell 30, that serves as the RF signal ground for RF center pin 10. The RF ground-providing shell 30 is configured and sized to be soldered within a step-shaped connector support bore 40 of the connector's support housing 50, and to project outwardly beyond a first surface 51 thereof. A forward or distal end 13 of the signal pin 10 projects into an interior hollow bore 35 of the conductive shell 30 which, like the pin 10, is preferably made of relatively low CTE conductive ferrous material, such as Kovar which has a coefficient of thermal expansion of substantially 5.2 PPM/° C., so that it and the pin may be readily hermetically bonded to the glass/dielectric spacer 20, which has a similarly low CTE, that is compatible with that of Kovar.
The step-shaped bore 40 of the housing 50 extends from the first surface 51 thereof to a second, opposite surface 52 of the support housing, and includes respectively different diameter bore portions that are successively contiguous with one another and the first and second surfaces of the housing. In order to conform with the stepped configuration of the bore 40, the reduced diameter portion 31 of the shell 30 is sized to be inserted into and disposed adjacent to the interior surface of a reduced diameter portion 41 of the bore 40, so that a first, relatively narrow, cylindrical gap 45 is defined between the outer sidewall of the reduced portion 31 of the shell 30 and the interior surface of the reduced diameter portion 41 of the bore 40.
In addition, shell 30 has a relatively wide diameter portion 32, that adjoins the relatively narrow diameter portion 31 thereof, and forms a second, relatively thin, annular gap 46, that is contiguous with the first, relatively narrow, cylindrical gap 45, and is formed between the bottom surface of the relatively wide diameter portion 32 of the shell and the annular surface of a step portion 42 of the bore 40 that connects the reduced diameter portion 41 of the bore to a relatively wide diameter portion 43 thereof. The shell 30 is conductively and fixedly retained within the step-shaped bore 40 by means of solder joint 60. This solder joint is produced by flowing solder material into the gaps 45 and 46 from a ring or annular-shaped solder preform, that has been inserted into an annular cavity 65 formed between the outer sidewall of the relatively wide diameter portion 32 of the shell 30 and the inner sidewall of the relatively wide diameter portion 43 of the bore 40.
A second portion 14 of the connector's center pin 10 passes through a relatively narrow diameter portion 44 of the step-shaped bore 40, which extends between a relatively shallow, circular depression or counterbore 47, at the bottom of the reduced diameter portion 41 of the bore 40, and the second surface 52 of the housing 50, and terminates at an exterior end 15. Counterbore 57 serves as a break for solder travel, by increasing the solder's propagation distance, which reduces capillary action, so that the solder will not travel along the surface of the bottom of the reduced diameter portion 41 of the bore 40, but rather will remain confined within the gaps 46 and 45 forming solder joint 60.
FIG. 2 shows the architecture of a second type of conventional coaxial feed-through RF connector, the components of which are installed at a bottom portion of a threaded connector support bore that extends into the housing from a first surface thereof, so as to allow an associated externally threaded RF connector, such as one that terminates the end of a section of RF cable, to be screwed into the threaded surface of the bore and engage the RF signal center pin installed therein. As such, the conductive material of the housing forms part of the RF signal ground that surrounds the RF signal pin.
More particularly, like the first type of prior art coaxial RF connector shown in FIG. 1, the coaxial feed-through RF connector of FIG. 2 includes a longitudinal (Kovar) signal pin 10, which is colinear with the connector's longitudinal axis 12, and is hermetically bonded to a coaxial bore 21 of a generally cylindrical dielectric (glass) spacer 20. Rather than being hermetically bonded to an RF ground-providing metallic shell that projects outwardly from the support housing, as in the RF connector architecture of FIG. 1, the outer cylindrical surface of the glass spacer 20 of the RF connector of FIG. 2 is hermetically bonded to a surrounding metallic (e.g., Kovar) cylindrical ferrule 70. Ferrule 70, which serves as the RF signal ground, is installed within a cylindrical recess 80 beneath the bottom portion 91 of a threaded connector-support bore 90, that is formed (e.g., machined) into the housing 50 from the first surface 51 thereof. The forward or distal end 13 of the signal pin 10 projects from the glass spacer 20 into the interior hollow portion 95 of the threaded bore 90.
Similar to the relatively narrow diameter portion of the RF signal ground-providing shell of the coaxial feed-through RF connector of FIG. 1, the outer diameter of the ferrule 70 is slightly less than the diameter of the cylindrical recess 80, so that a relatively narrow, cylindrical gap 75 is formed therebetween. The RF signal ground-providing ferrule 70 of the connector of FIG. 2 is conductively and fixedly retained within recess 80 by means of solder joint 85 formed in the cylindrical gap 75. Solder joint 85 not only serves to physically affix the RF signal pin support structure within the housing, but provides an ohmic connection between the Kovar ferrule 70 and the aluminum housing 50, so that the housing provides part of the RF signal ground surrounding the RF signal pin 10. The solder joint 85 is produced by flowing solder material into the cylindrical gap 75 from a ring or annular-shaped solder preform, that has been inserted into an annular depression 66 that is contiguous with the cavity 80 and the bottom portion 91 of the threaded bore 90.
Also similar to the RF connector of FIG. 1, in the RF connector architecture of FIG. 2, a second portion 14 of the RF signal pin 10 passes through a relatively narrow diameter bore 100 in the housing 50, which extends between a relatively shallow, circular counterbore 102 at the bottom of the recess 80 and the second surface 52 of the housing, and terminates at an exterior end 15. As described previously, such a counterbore effectively prevents solder from traveling along the bottom of the recess 80, so that the solder remains confined within the relatively narrow, cylindrical gap 75, forming the intended solder joint 85.
In each of the coaxially configured RF connectors shown in FIGS. 1 and 2, the only reliable hermetic seals are those provided by the hermetic bond between the Kovar RF center pin and the glass spacer, and the hermetic bond between the glass spacer and the Kovar material of a surrounding RF signal ground-providing cylinder (shell 30 in FIG. 1, and ferrule 70 in FIG. 2). On the other hand, the solder joint that has been formed between the Kovar ferrule and the aluminum housing can be expected to suffer cyclic fatigue, producing cracks that will propagate and cause the solder joint to lose whatever temporary hermeticity it may have possessed when initially formed. This failure of such a solder joint is due to the substantial mismatch between the CTEs of Kovar and aluminum.
Still, if the connectors are relatively small sized, and the solder joints between metals having substantially different CTEs are formed in a dependable and repeatable manner, the types of connectors shown in FIGS. 1 and 2 are sometimes considered to be ‘sufficiently’ hermetically sealed, so as to conform with some industry standards. Namely, in some applications that require a hermetically sealed connector, the connectors of FIGS. 1 and 2, which are not reliably hermetically sealed structures, may be employed as an alternative to the preferred device.
One prior art approach to resolve the above-described CTE mismatch problem, that leads to solder joint fatigue and loss of any hermeticity that the solder joints of an RF connector may initially provide, involves laser-welding the RF signal ground-providing Kovar ferrule, to which the glass spacer supporting the Kovar RF signal pin is hermetically bonded, to a coaxial sleeve made of a dissimilar metal (e.g., aluminum), that has the same CTE as the (aluminum) support housing. The coaxial sleeve is made of Kovar and aluminum. Kovar ferrule is welded to Kovar portion of coaxial sleeve and the dissimilar metal (aluminum) coaxial sleeve is laser welded to a connector retention bore in the aluminum housing. One portion of the coaxial sleeve has the same CTE as the ferrule and the other portion of the sleeve has the same CTE as the housing. The sleeve is a transition joint for the Kovar feed thru to the aluminum housing. In such an alternative RF connector structure, the laser welds, which form individual hermetic seals, make up for the lack of reliable hermeticity of the solder joints employed in the RF connector architectures of FIGS. 1 and 2, so that the resulting RF connector is completely and reliably hermetically sealed to the aluminum support housing.
An example of a prior art RF connector architecture employing such laser-welds to hermetically join a dissimilar metal coaxial sleeve to the RF signal ground-providing (Kovar) cylinder surrounding the (Kovar) center pin, and to hermetically join the dissimilar metal coaxial sleeve to a connector retention bore in the (aluminum) housing, is diagrammatically illustrated in FIG. 3. As shown therein, like the coaxial feed-through RF connectors of FIGS. 1 and 2, the coaxial feed-through RF connector of FIG. 3 has a longitudinal (Kovar) RF signal pin 10 disposed along the connector's longitudinal axis 12, and hermetically bonded to a coaxial bore 21 of a generally cylindrical dielectric (glass) spacer 20. The glass spacer 20 abuts against the bottom portion 109 of an electrically conductive grounding spring 110. Grounding spring 110 is installed at the bottom 112 of a cylindrical recess 114 that is contiguous with and extends beneath the bottom portion 116 of a bore 120 formed into the housing 50 from top surface 51.
The forward or distal end 13 of the RF signal pin 10 projects from the glass spacer 20 into a hollow interior portion 122 of a threaded interior surface 124 of a cylindrical sleeve 125. Cylindrical sleeve, 125 includes a first, metallic sleeve portion 126, made of a metal (e.g., aluminum) that may be readily metallurgically joined with (e.g., welded) by way of a (laser) weld joint 132 to the metal (e.g., aluminum) of the housing 50. Sleeve 125 further includes a second, metallic sleeve portion 127, that adjoins the first sleeve portion 126, and is made of a metal, such as Kovar, that may be readily (laser) welded at 133 to a metallic (e.g. Kovar) ferrule 128, which is coaxially adjacent to the second sleeve portion 127. The first, metallic sleeve portion 126 is metallurgically joined to the second, metallic sleeve portion 127 by way of an explosion weld joint 130 therebetween. Kovar ferrule 128, has a lower projection portion 129 and is hermetically bonded to the outer surface of the glass spacer 20. In the connector's installed position, the lower projection portion 129 of the Kovar ferrule 128 is urged against the bottom portion 109 of the grounding spring 110, so that the bottom portion 109 of the grounding spring 110 is firmly captured between the Kovar ferrule 128 and the bottom 112 of the bore 120. In addition, an upper portion 111 of the grounding spring 110 abuts against a bottom surface 131 of the Kovar ferrule 128. As a consequence, the grounding spring 110 provides a secure RF ohmic signal ground connection between the Kovar ferrule 128 and the conductive material of housing 50.
The outer diameters of the sleeve 126 and the ferrule 128 are slightly less than the diameter of the cylindrical bore 120, so that, once the Kovar sleeve portion 127 of sleeve 125 and the Kovar ferrule 128 have been welded together at laser weld joint 133, they may be readily inserted into the cylindrical bore 120. After being inserted into the bore 120, the combined (explosion-welded) sleeve and ferrule structure is hermetically sealed with the aluminum of the surrounding housing, by laser-welding the (aluminum) sleeve portion 126 of the sleeve 125 to the adjoining portion of the surface 51 of the (aluminum) housing 50, so as to produce laser-weld joint 132 therebetween.
Now, although explosion- and laser-welds, such as those employed in the coaxial RF connector architecture of FIG. 3, may be employed to form hermetic seals between RF connector components having dissimilar CTEs, and thereby remedy problems associated with the use of solder joints, such as the formation and propagation of cracks in the joints as result of cyclic fatigue, the processing techniques necessary to form such welds are relatively complicated, which makes the connectors expensive and often increase the size of the connectors.